1. Field of the Invention
This invention relates to seismic sources and more specifically to methods for controlling the signature of a marine seismic source.
2. Discussion of the Prior Art
To obtain information on substrata located below a body of water, seismic sources adapted for generating an acoustic signal in the water are towed through the water by towing vessels. The acoustic signal generated in the water travels in all directions, and part of this acoustic energy, after having been reflected or refracted by the substrata, returns to the body of water overlying the substrata and is detected by the hydrophones of a detector cable that is towed through the water in the neighborhood of the seismic source.
The marine sources most commonly used today are impulsive sources such as the air gun. Air guns, which generate an acoustic wave by the sudden release of a compressed gas into the water, also generate a train of bubble pulses. When the compressed gas is released from the air gun, a gas bubble expands outwardly from the source until the pressure inside the bubble reduces to the point that the hydrostatic pressure of the water causes the bubble to contract. This contracton of the air bubble increases the air pressure within the air bubble again until the internal air bubble pressure exceeds the hydrostatic pressure and the bubble again expands, thereby causing a secondary acoustic wave called a bubble pulse. Normally, a series of these bubble pulses will be emitted.
Use a single impulsive source, or a plurality of substantially identical sources fired simultaneously, results in a seismic signal whose frequency spectrum exhibits peaks and notches related to the bubble pulse oscillation period. FIG. 2A illustrates the signature from such a plurality of substantially identical sources and FIG. 2B shows the corresponding frequency spectrum of that signature.
It is a common practice in the industry to utilize an array of air guns of different sizes with different oscillation periods in order to produce a seismic signal having a flatter frequency spectrum. A typical signature from such a "tuned" array is shown in FIG. 3A, and its frequency spectrum is shown in FIG. 3B.
Although a broadband frequency spectrum such as shown in FIG. 3B is generally considered desirable, in some areas a seismic signal having energy concentrated in a selected frequency range would be more useful. In some offshore regions, the higher fequencies do not penetrate to the depths of interest, but instead are trapped in the water and near-surface earth layers. Such trapped energy creates strong multiples that obscure the useful signals from lower layers.
It is an object of this invention to generate a seismic source signal whose frequency content is concentrated in the range below a selected frequency. It is also an object of this invention to smooth the frequency spectrum of the signal.